Tire With Optimised Tread

ABSTRACT

A tire, the speed rating of which is at least V, comprises a tread, having a volumetric void ratio at least equal to 2% and at most equal to 20%, having a central part Pc comprising at least one groove, and two lateral parts PI each having a volumetric void ratio at most equal to 2%. The central part Pc of the tread has an axial width at most equal to one third of the axial width LBDR of the tread, and any portion of the central part Pc of the tread that is circumferentially delimited by two meridian planes separated by a circumferential distance dc equal to one tenth of the outer perimeter P of the tire comprises at least one and at most three grooves that open onto the meridian planes delimiting the portion in question.

FIELD OF THE INVENTION

The present invention relates to a passenger vehicle tire, the speedrating of which is at least V, that is to say a tire that is intended tobe mounted on a vehicle, for example a competition vehicle, of which themaximum speed is at least equal to 240 km/h. The invention relates moreparticularly to the tread of such a tire. The expression “speed ratingthat is at least V” is understood to mean all tires of which the speedrating allows a maximum speed at least equal to 240 km/h.

Since a tire has a geometry that exhibits symmetry of revolution aboutan axis of rotation, the geometry of the tire is generally described ina meridian plane containing the axis of rotation of the tire. For agiven meridian plane, the radial, axial and circumferential directionsdenote the directions perpendicular to the axis of rotation of the tire,parallel to the axis of rotation of the tire and perpendicular to themeridian plane, respectively. A circumferential plane is a planeperpendicular to the axis of rotation. The median circumferential plane,referred to as equator, is the plane perpendicular to the axis ofrotation and passing through the middle of the tread.

In the following text, the expressions “radially on the inside of” and“radially on the outside of” mean “closer to the axis of rotation of thetire, in the radial direction, than” and “further away from the axis ofrotation of the tire, in the radial direction, than”, respectively. Theexpressions “axially on the inside of” and “axially on the outside of”mean “closer to the equatorial plane, in the axial direction, than” and“further away from the equatorial plane, in the axial direction, than”,respectively. A “radial distance” is a distance with respect to the axisof rotation of the tire and an “axial distance” is a distance withrespect to the equatorial plane of the tire. A “radial thickness” ismeasured in the radial direction and an “axial width” is measured in theaxial direction.

A tire comprises a crown comprising a tread that is intended to comeinto contact with the ground via a tread surface, two beads that areintended to come into contact with a rim, and two sidewalls that connectthe crown to the beads. Furthermore, a tire comprises a carcassreinforcement, comprising at least one carcass layer, radially on theinside of the crown and connecting the two beads.

The tread of a tire is delimited, in the radial direction, by twocircumferential surfaces, the radially outermost of which is referred toas the tread surface and the radially innermost of which is referred toas the wear limit surface. In addition, the tread of a tire isdelimited, in the axial direction, by two lateral surfaces. The tread isalso made up of one or more rubber compounds, also referred to asrubbers. The expression “rubber compound” refers to a composition ofrubber comprising at least an elastomer and a filler.

Passenger vehicles or competition vehicles of which the maximum speed isat least equal to 240 km/h can be used on racing circuits. In order toobtain maximum grip performance on dry ground, these vehicles areequipped with slick tires, i.e. ones without cuts in the tread. For useon wet roads, the tires are provided with cuts that open in particularonto the lateral surfaces of the tread in order to evacuate the watercontained in the contact patch between the tire and the ground on whichit runs.

A cut denotes either a well, or a groove, or a sipe, or acircumferential furrow, and forms a space opening onto the treadsurface. A well has, on the tread, an opening surface inscribed in acircle of characteristic diameter W. A sipe or a groove has, on thetread surface, two characteristic main dimensions: a width W and alength Lo, such that the length Lo is at least equal to twice the widthW. A sipe or a groove is therefore delimited by at least two mainlateral faces that determine its length Lo and are connected by a bottomface, the two main lateral faces being at a non-zero distance from oneanother referred to as the width W of the sipe or of the groove. In thecase of a groove of variable width, it is possible for a person skilledin the art to calculate a mean width of the groove.

PRIOR ART

In the context of competition, the grip performance on dry ground isfundamental. This objective is traditionally achieved by enlarging theground contact surface area in order to maximize the contact forcesbetween the tread and the ground on which it runs in the contact patch;this can be achieved by reducing the internal pressure of the tire, byoptimizing the architecture of the tire, but it is impossible to reducea volumetric void ratio which, by definition, is zero, that is to say ina tread without cuts.

Furthermore, when the governing body of the motor sport “formula 1”decided to limit the grip of tires, it obliged tire manufacturers toprovide several circumferential furrows, in this case 4, distributedacross the entire width of the tread of the tire in order to limit theamount of rubber in contact with the ground and thus the grip on dryground.

However, there is an increasing need for grip that depends on thedifferent racing regulations. These same rules limit the sizes of thetires. The solutions usually consist in varying the rubber compounds, orrubbers, of which the tread is made. However, the rubbers with the bestgrip often cause problems in terms of rate of wear since they are lessstiff and wear away quickly.

In addition, in this specific context, the need for grip is greater athigh speed, for braking at maximum speed on entering bends and not forstopping.

SUMMARY OF THE INVENTION

The main objective of the present invention is therefore to increase thegrip of a tire, the speed rating of which is at least V, that is to sayone that is intended to be mounted on a vehicle of which the maximumpermissible speed is at least equal to 240 km/h, the axial width ofwhich is at least equal to 200 mm, and having an outer perimeter P,comprising:

-   -   a tread, intended to come into contact with the ground via a        tread surface, having an axial width LBDR and a volumetric void        ratio at least equal to 2% and at most equal to 20%,    -   the tread having a central part Pc, centered on an equatorial        plane C and delimited by two circumferential planes (C1, C2),        and two lateral parts PI positioned axially on either side of        the central part Pc, each lateral part PI having a volumetric        void ratio at most equal to 2%,    -   the central part Pc of the tread comprising at least one groove,        forming a space opening onto the tread surface and being        delimited by at least two main lateral faces connected by a        bottom face,    -   the at least one groove in the central part Pc of the tread        having a mean width W at least equal to 6 mm and at most equal        to 30 mm, preferably at most equal to 20 mm, and a depth H at        least equal to 3 mm and at most equal to 6 mm,    -   the central part Pc of the tread has an axial width at most        equal to one third of the axial width LBDR of the tread,    -   any portion of the central part Pc of the tread that is        circumferentially delimited by two meridian planes separated by        a circumferential distance dc equal to one tenth of the outer        perimeter P of the tire comprises at least one and at most three        grooves that open onto the meridian planes delimiting the        portion in question.

The invention is based on the use of the aerodynamics of the tire. Theseaerodynamics only become influential for high-speed uses. It is for thisreason that the invention relates to tires of which the speed rating isat least V or which are intended to be mounted on a vehicle of which themaximum speed is at least 240 km/h.

High-speed running of the tire causes raised pressure of the air outsidethe tire on entering the contact patch of the tire with respect to therunning direction and negative pressure of this air on leaving thecontact patch. The wider the tire, the greater this difference inpressure is. For a tire with a small width, this difference in pressureis not exploitable. Therefore, the invention relates to tires of whichthe axial width, measured between the axially outermost points of thetire, is at least equal to 200 mm.

By positioning a continuous groove in the tread between the start of thecontact patch and the end, a flow of air is created which brings about asuction effect of the tire, which creates the equivalent of anaerodynamic load as would be made by a spoiler fitted on the vehicle.One of the advantages of this solution is that adding a spoilerincreases the drag of the vehicle and thus the fuel consumption thereof,whereas the presence of a groove in the tread that allows the air toflow from the start to the end of the contact patch does not increasethe drag of the vehicle.

The raised pressure and the negative pressure are at a maximum at theequatorial plane of the tire and decrease away from the latter, suchthat the groove(s) present in the tread should be advantageouslypositioned in the central part of the tread. This central part iscentered on the equatorial plane of the tire and has an axial widthequal to one third of the width of the tread (LBDR/3).

The groove(s) advantageously have a mean width at least equal to 6 mm inorder to ensure that they do not close on passing through the contactpatch. The mean width of the at least one groove is at most equal to 30mm, preferably 20 mm, so as not to excessively reduce the area of thecontact patch in order to ensure correct performance in terms of gripand behaviour at all speeds of the vehicle on which the tires aremounted. In addition, for this same objective, the volumetric void ratioof the tread should be at most equal to 20%, preferably at most equal to12%.

If there are more than three grooves in the central part of the tread,the raised pressure and the negative pressure are reduced, as is therate of flow in the grooves. As a consequence, the suction effectdecreases and no longer compensates the loss of rubber in contact withthe ground as a result of the presence of the grooves.

In order to realize the invention, the grooves should be disposed aroundthe entire circumference of the tread of the tire, in the central partof the tread such that, regardless of the portion of the tread surfacein contact with the ground, at least one groove and at most threegrooves allow the air to flow from the start of the contact patch of thetire to the end of the contact patch. For this type of tire, in thehigh-speed phases in which the effect occurs, the contact patchrepresents at most one tenth of the circumference of the tire. In orderto realize the invention, all that is thus necessary is to dispose thegrooves such that any portion of the tread contained between twomeridian planes, the circumferential distance (dc) of which is equal toone tenth of the outer perimeter of the tire, comprises at least one andat most three grooves that open onto the meridian planes delimiting theportion in question.

A groove is said to open onto a plane if the intersection between theplane and the groove is not zero. If the intersection between the grooveand the two planes is not zero, the air can flow through the groove fromone plane to the other. In addition, the circumferential distancebetween said meridian planes mentioned in the invention is such that itis at least equal to the circumferential length of the contact patch.Therefore, regardless of the circumferential position of the contactpatch on the tire, there is a groove that allows air to flow between theraised pressure and the negative pressure on either side of the contactpatch, making it possible, at high speed, to create the suction effectdesired by the invention.

Given the grip performance intended for the tire, it is not advisablefor the material that is in contact with the ground and thus forms atleast a part of the tread not to have a high level of grip. The grip ofa rubber material may in particular be characterized by the loss at 60°C., measured at 60° C. and 10 Hz. In a preferred solution, the treadcomprises a rubber material, the loss factor of which, measured at 60°C. and 10 Hz, is at least equal to 25%, preferably at least equal to35%.

The loss factor at 60° C., which is a loss of energy at 60° C. byrebound at a set energy level measured at the sixth impact and the valueof which, expressed in %, is the difference between the energy suppliedand the energy returned, divided by the energy supplied.

The tires in question are also designed to have high cornering stiffnessand lateral stiffness in order to ensure a very good performance interms of behaviour: the radial thickness of the tread is thus limited to7 mm and, for an improved performance, limited to 5.5 mm. Since theinvention requires the existence of a groove with a depth at least equalto 3 mm, the radial thickness of the tread is therefore at least equalto 3 mm. A preferred solution is for the tread to have a maximum radialthickness E at least equal to 3 mm and at most equal to 7 mm, preferablyat most equal to 5.5 mm.

Given the distribution of raised pressures and negative pressures at thestart and end of the contact patch with respect to the equatorial plane,a preferred solution is for the central part Pc, in which the groovesare disposed, to be centered on the equatorial plane and to have anaxial width equal to one quarter of the tread width (LBDR/4), preferablyto one fifth thereof (LBDR/5).

It is advantageous for the respective void ratios of the lateral partsPI of the tread to be zero. This optimizes the contact patch of the tireand improves grip. The measurement of the volumetric void ratio does nottake into account any devices for evaluating the wear limit of the tireon account of the wear in the tread, such as wear wells.

In the case of tires designed to be subject to high speeds, such as thetires according to the invention, it is common for the tires to have apredetermined direction of mounting on the vehicle. In this case, eachtire has an outer axial edge and an inner axial edge. The inner axialedge is the edge intended to be mounted on the bodyshell side of thevehicle when the tire is mounted on the vehicle in said predetermineddirection of mounting. In one advantageous solution, at least one grooveof the tread has an inner main lateral face, axially closest to theinner axial edge, that forms a mean taper angle (AI) with acircumferential plane (XZ) that is at least equal to 10°, preferably atleast equal to 30°. This solution makes it possible to defend againstlocalized forms of wear of the tread at the lateral face of the at leastone groove axially closest to the inner axial edge.

Although the outer lateral face of a groove, that is to say the oneaxially furthest away from the inner axial edge, referred to as theouter lateral face, is less sensitive to local wear, it may likewise beadvantageous for said outer lateral face to form what is known as ataper angle with the radial axis. Given the asymmetries associated withthe use of vehicles, one preferred solution is for the at least onegroove in the tread to have an outer main lateral face, axially furthestaway from the inner axial edge, that forms a mean taper angle (AE) witha circumferential plane (XZ) that is at least equal to the mean taperangle (AI) of the inner main lateral face.

The tires according to the invention can be used on closed circuits withthe scope of motor racing competitions. In these competitions, the tiresused can have grooves created by moulding, during the curing of thetire, or tires that may be cut by any mechanical cutting or machiningmethod so as to have this type of groove. Cutting of the tire is in factcarried out in motorsports in order to adapt to the specific useconditions on the day of the competition.

In one advantageous solution, the central part Pc only containssubstantially circumferential grooves around the entire circumference ofthe tread, in other words circumferential furrows. This solution is infact the simplest embodiment of the invention. The invention also workswith one, two or three substantially circumferential grooves.Advantageously, the central part Pc of the tread contains a single,substantially circumferential groove extending around the entirecircumference of the tread.

It is advantageous for the axial positions of the lateral faces of thesubstantially circumferential groove to have variations (v) in thecircumference at least equal to 1 mm with respect to their mean axialpositions. These variations can thus create local narrowings in thegroove by locally decreasing the section thereof in order to locallycreate a Venturi effect that will increase the rate of air flow and thusthe association suction effect. These variations in axial position canalso be coordinated between the two lateral faces of the grooves. Thismakes it possible to increase the shear stiffness of the tread aroundthe grooves and thus to increase stiffness over the tire, this being aneffect frequently desired in this type of tire.

It is thus particularly advantageous for the mean width W of the single,substantially circumferential groove to have variations (v) at leastequal to 1 mm around the entire circumference of the tire.

It is advantageous for the wear limit of the tire as a result of wear tobe indicated only with the aid of devices disposed in the tread awayfrom the grooves in the central part Pc of the tread. Specifically, thepresence of these devices, referred to as wear indicators, in thegrooves can disrupt the flow and, depending on the disposition andgeometry of said devices, reduce the effectiveness of the invention.

The vehicles on which the intended tires are mounted often do not haveshielding around the mounted assemblies so as not to increase the weightof the vehicle. The term mounted assembly means, in technical language,a tire mounted on a rim.

These vehicles for which the acceleration performance of which isparamount are often likewise provided with aerodynamic spoilers in frontof at least one of the mounted assemblies of the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and other advantages of the invention will be understoodbetter with the aid of FIGS. 1 to 8, said figures not being drawn toscale but in a simplified manner so as to make it easier to understandthe invention:

FIG. 1 is an overall view of the tire 1 according to the invention, inparticular the tread 2 thereof and the tread surface 21 thereof. Thetire in question is provided with a mounting direction SM indicated forexample on the sidewall, but able to be indicated as desired by themanufacturer's.

FIG. 2 shows a meridian section through the crown of a tire according tothe invention and illustrates the central part Pc, the lateral parts PI,the width LBDR of the tread, the grooves 22 and the two main lateralfaces 221 and 222 thereof that are connected by a bottom face 223.

FIGS. 3A and 3B show two types of radially exterior meridian profile ofthe tread 2 of a passenger vehicle tire, for which the measurement ofthe width of the tread is specified.

FIG. 4 defines the “inner axial edge” 46 and “outer axial edge” 45 of atread.

FIG. 5 illustrates a disposition of grooves 22 according to theinvention.

FIG. 6 illustrates a first embodiment of a tread according to theinvention with a single circumferential groove 22, or circumferentialfurrow, the lateral faces 221 and 222 of which have variations in axialposition (v) in the circumferential direction at least equal to 1 mmwith respect to their mean axial position in order to create localVenturi effects.

FIG. 7 illustrates a second embodiment of a tread according to theinvention with a single circumferential groove 22, or circumferentialfurrow, the lateral faces 221 and 222 of which have variations in axialposition (v) in the circumferential direction at least equal to 1 mmwith respect to their mean axial position in order to stiffen the tirearound the lateral faces of the groove.

FIG. 8 illustrates the mean taper angle AI of the inner lateral face ofa groove 22, axially closest to the inner axial edge 46 intended to bepositioned on the inner side of the vehicle, and the mean taper angle AEof the outer lateral face of a groove 22, axially furthest away from theinner axial edge 46.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a tire 1 having a tread 2 and aoptionally a mounting direction SM recommended by the manufacturer. Thetread has a tread surface 21. FIG. 1 also shows the frame of reference(O, X, Y, Z) used.

FIG. 2 schematically shows a meridian section, in a meridian plane YZ,through the crown of a tire according to the invention. It illustratesin particular the width LBDR and the thickness E of the tread 2, andalso the central part Pc and lateral parts PI thereof. Also shown are agroove 22 and the lateral faces 221 and 222 and the bottom face 223thereof. The depth H of a groove 22 is the radial distance between theradially innermost point of the bottom surface 223 and the point of thetread 21 that is closest to said point. FIG. 2 also shows the mean widthW of the groove 22.

In FIGS. 3A and 3B, the axial ends 7 of the tread, which make itpossible to measure the tread width in this meridian plane, aredetermined in a meridian plane. In FIG. 3A, in which the tread surface21 intersects the outer axial surface of the tire 8, the axial border 7is determined trivially by a person skilled in the art. In FIG. 3B, inwhich the tread surface 21 is continuous with the outer axial surface ofthe tire 8, the tangent to the tread surface at any point on said treadsurface in the region of transition towards the sidewall is plotted on ameridian section of the tire. The first axial border 7 passes throughthe point for which the angle β (beta) between said tangent and an axialdirection YY′ is equal to 30°. When, in a meridian plane, there areseveral points for which the angle β between said tangent and an axialdirection YY′ is equal to 30°, it is the radially outermost point thatis taken into account. The same method is followed to determine thesecond axial end of the tread surface. The width of the tread, in themeridian plane, is the axial distance between the two points of themeridian plane of the two axial ends of the tread surface. The width ofthe tread of the tire is the maximum value of the widths of the treadover all the meridians.

FIG. 4 schematically shows tires mounted on mounting rims of wheels of avehicle 200 and having a predetermined direction of mounting on thevehicle. Each tire has an outer axial edge 45 and an inner axial edge46, the inner axial edge 46 being the edge mounted on the bodyshell sideof the vehicle when the tire is mounted on the vehicle in saidpredetermined direction of mounting, the outer axial edge 45 being theopposite of that. In the document, the expression “outboard side of thevehicle” denotes the outer axial edge 45.

FIGS. 5 to 7 show various embodiments of a tread according to theinvention. FIG. 5 shows a tread, the central part Pc of which hasseveral grooves 22 such that any portion of the central part Pc of thetread contained between two meridian planes (M1, M2), thecircumferential distance dc of which is at most equal to one tenth ofthe outer perimeter of the tire P, comprises at least one and at mostthree grooves 22 that open onto the meridian planes (M1, M2) delimitingthe portion in question, and are closed on the circumferential planes(C1, C2) delimiting the central part of the tread.

FIGS. 6 and 7 show two embodiments of a tread according to the inventionwith a single, substantially circumferential groove, or circumferentialfurrow, in the central part Pc. FIG. 6 presents variations (v) in theaxial positions of the lateral faces 221 and 222 of the groove 22,creating variations in the width W of the groove that are likely tocreate Venturi effects in order to accentuate the suction effect of theinvention, according to a first embodiment of the invention. This alsoincreases the shear stiffness of the tread around the groove.

FIG. 7 presents variations (v) in the axial positions of the lateralfaces 221 and 222 of the groove 22, without any variation in the width Wof the groove, according to a second embodiment of the invention. Thismakes it possible to increase the shear stiffness of the tread aroundthe groove without creating a Venturi effect, since the width W of thegroove remains constant around the circumference. Stiffening the treadpattern in this way makes it possible to improve the performance interms of behaviour and of wear of the tire.

FIG. 8 shows, according to a third embodiment of the invention, theangles AI and AE, referred to as taper angles, of the inner lateral face221 and of the outer lateral face 222, respectively, depending on theirrespective positions with respect to the inner axial edge 46 and outeraxial edge 45 of the tire. The inner lateral face 221 is axially closerto the inner axial edge 46 of the tire. The taper angle of a lateralface of a groove is measured in the meridian plane between the radialaxis and the tangent to the lateral face of the groove at the medianradial coordinate point, between the tread surface and the radiallyinnermost point of the bottom face of the groove in the meridian planein question. The figure is in accordance with the invention;specifically, the taper angle AI of the inner lateral face 221 is atleast equal to the taper angle AE of the outer lateral face 222.According to this embodiment of the invention, the taper angle AE is atmost equal to the taper angle AI.

The inventors made calculations and carried out tests on the basis ofthe invention for a passenger vehicle tire of size 245/40 R18, inflatedto a pressure of 1.7 bar in the cold state, with an axial width of 245mm. They compared the tire A according to the prior art, with no groovesat all in its tread, with the tire B according to the invention, in thetread of which there are, in the central part Pc thereof, twocircumferential grooves with a width of 10 mm and a depth of 4 mm, for avoid ratio of 11%. The tread has a thickness of 5.7 mm. There are noother grooves or cuts in the lateral parts PI of the tread, the voidratio of which is zero.

The tread comprises a rubber material, the loss factor of which,measured at 60° C. and 10 Hz, is equal to 54%. The taper angles of thelateral faces of the grooves are zero.

The grooves were obtained by cutting a control tire A.

The calculations showed a change in pressure in the contact patch thatis able to be observed in a high-speed test. Positioning the grooves inthe central part of the tread makes it possible, from the calculations,to double the pressure difference effect compared with positioning thegrooves in the lateral parts.

The tires were tested during braking on one and the same motor racingvehicle. Said racing vehicle was tested with a set of tires A, referredto as the control set, and then with a set of tires B according to theinvention. The result given is the average of three tests.

For braking at low speed of between 100 km/h and 5 km/h, the reductionin the contact area on account of the presence of the grooves is visibleand the braking distance for the set of tires B according to theinvention increases by more than 2.5% compared with the control set oftires A, this corresponding to a deterioration in the grip performance,but in a range of speeds that is of little interest on a motor racingcircuit.

By contrast, in a braking test at a high speed of between 250 km/h and150 km/h, the braking distance for the set of tires B according to theinvention decreases by more than 0.8% compared with the control set oftires A, this corresponding to an improvement in the grip performance,in a range of speeds that is of interest on a motor racing circuit.

1. A tire, the speed rating of which is at least V, that is to say onethat is adapted to be mounted on a vehicle of which the maximumpermissible speed is at least equal to 240 km/h, the axial width ofwhich is at least equal to 200 mm, and having an outer perimeter P,comprising: a tread, adapted to come into contact with the ground via atread surface, having an axial width LBDR and a volumetric void ratio atleast equal to 2% and at most equal to 20%; the tread having a centralpart Pc, centred on an equatorial plane C and delimited by twocircumferential planes, and two lateral parts PI position axially oneither side of the central part Pc, each lateral part PI having avolumetric void ratio at most equal to 2%; the central part Pc of thetread comprising at least one groove, forming a space opening onto thetread surface and being delimited by at least two main lateral facesconnected by a bottom face; the at least one groove in the central partPc of the tread having a mean width W at least equal to 6 mm and at mostequal to 30 mm, and a depth H at least equal to 3 mm and at most equalto 6 mm, wherein the central part Pc of the tread has an axial width atmost equal to one third of the axial width LBDR of the tread, andwherein any portion of the central part Pc of the tread that iscircumferentially delimited by two meridian planes separated by acircumferential distance dc equal to one tenth of the outer perimeter Pof the tire comprises at least one and at most three grooves that openonto the meridian planes delimiting the portion in question.
 2. The tireaccording to claim 1, wherein the tread comprises a rubber material, theloss factor P60 of which, measured at 60° C. and 10 Hz, is at leastequal to 25%.
 3. The tire according to claim 1, wherein the treadcomprises a rubber material, the loss factor P60 of which, measured at60° C. and 10 Hz, is at least equal to 35%.
 4. The tire according toclaim 1, wherein the tread has a maximum radial thickness E at leastequal to 3 mm and at most equal to 7 mm.
 5. The tire according to claim4, wherein the tread has a maximum radial thickness E at most equal to5.5 mm.
 6. The tire according to claim 1, wherein the central part Pc ofthe tread has an axial width equal to one quarter of the axial widthLBDR of the tread.
 7. The tire according to claim 1, wherein therespective void ratios of the lateral parts PI of the tread are zero. 8.The tire according to claim 1, comprising a tread having two axialedges, one of which, positioned on the inner side of the vehicle, isreferred to as the inner axial edge, wherein at least one groove in thetread has an inner main lateral face, axially closest to the inner axialedge, that forms a mean taper angle with a circumferential plane that isat least equal to 10°.
 9. The tire according to claim 8, wherein themean taper angle of the inner main lateral face is at least equal to30°.
 10. The tire according to claim 8, wherein the at least one groovein the tread has an outer main lateral face, axially furthest away fromthe inner axial edge, that forms a mean taper angle with acircumferential plane that is at most equal to the mean taper angle ofthe inner main lateral face.
 11. The tire according to claim 1, whereinat least one groove in the central part Pc of the tread is obtained bymoulding.
 12. The tire according to claim 1, wherein at least one groovein the central part Pc of the tread is obtained by a mechanical cuttingor machining operation.
 13. The tire according to claim 1, wherein thecentral part Pc of the tread contains a single, substantiallycircumferential groove extending around the entire circumference of thetread.
 14. The tire according to claim 13, wherein the mean width W ofthe single, substantially circumferential groove has variations (v) atleast equal to 1 mm around the entire circumference of the tire.